A retarder is an admixture that slows down the chemical process of hydration so that the concrete remain plastic and workable for a longer time than concrete without retarder.The retarders are used in casting and compacting or consolidating a large number of pours without formation of cold joints.

Some times concrete may have to be placed in difficult conditions and delay may occur in transporting and placing.In ready mixed concrete practices, concrete is manufactured in central batching plant and transported over a long distance to the job site which may take considerable time.In the above cases,the setting of concrete will have to be retard so that concrete will finally placed and compacted is in perfect plastic state.

The most commonly known retarder is calcium sulphate. It is inter ground to retard the setting of cement.The approprite amount of gypsum to be used must be determined carefully for the given job. otherwise excess amount may cause undesirable expansion and indefinite delay in the setting of concrete.

In addition to gypsum  there are no of materials found to be suitable for the purpose

  1. starches
  2. cellulose products
  3. sugars
  4. acids or salt of acids

Common sugar is one of the most effective retarding agents used as an admixture for delaying the setting time of concrete,without detrimental effect to the ultimate strength.